\documentclass{standalone}
\usepackage{tikz}
\usepackage{ctex,siunitx}
\setCJKmainfont{Noto Serif CJK SC}
\usepackage{tkz-euclide}
\usepackage{amsmath}
\usepackage{wasysym}
\usetikzlibrary{patterns, calc}
\usetikzlibrary {decorations.pathmorphing, decorations.pathreplacing, decorations.shapes}
\begin{document}
\small
\begin{tikzpicture}[>=latex,yscale=1.6]
  % \useasboundingbox(0.9,0)rectangle(5.1,5);
  \draw[decorate,decoration=brace, thick](3.55,1.4796)--node[right]{$i_b$}(3.55,0);
  \draw[decorate,decoration=brace, thick](3.55,0)--node[right]{$i_c$}(3.55,-0.9534);
  \draw[decorate,decoration=brace, thick](3.45,-.5262)--node[left]{$i_a$}(3.45,0);
  \draw[->] (0,-1.6)--(0,1.8)node [right]{$i$};
  \draw [->](0,0)--(7,0)node [right]{$t$};
  \draw [thick] plot [domain=0:3.1416*2, samples=1000] function{1.5*sin(x)} ;
  \draw [very thick] plot [domain=0:3.1416*2, samples=1000] function{1.5*sin(3.1416*2/3+x)} ;
  \draw [ultra thick] plot [domain=0:3.1416*2, samples=1000] function{1.5*sin(3.1416*4/3+x)} ;
  \node at (3.1416/2, 1.7){$i_a$};
  \node at (3.1416/2+3.1416*2/3, 1.7){$i_b$};
  \node at (3.1416/2+3.1416*4/3, 1.7){$i_c$};
  \draw[dashed](3.5, 1.4796)--(3.5,-0.9534);
\end{tikzpicture}
\end{document}